The disorder of self-injurious behavior in monkeys is characterized with occasional bouts of wounding and frequent episodes of non-injurious biting Little is known about the functions of such behavior in rhesus monkeys; however, this disorder may persist because of some reinforcing quality such as tension reduction Evidence from human populations with this disorder (e g , prisoners who engage in cutting) provide strong support for this view To examine the tension reduction hypothesis in monkeys, we analyzed heart rate activity during episodes of non-injurious, self-directed biting (NIB) The subjects were 7 monkeys involved in heart rate testing that were observed to bite themselves (NIB) during the test The monkeys were fitted with vests containing radiotelemetry units in the afternoon and monitored for heart rate the next morning The monkeys were simultaneously videotaped to record their behavior Heart rate was analyzed in 30-second epochs We evaluat ed heart r ate in intervals prior to, during, and just after a biting episode and compared these values to the baseline heart rate Heart rate was significantly elevated during biting episodes compared to the intervals prior to and after biting Heart rate had already begun to increase in the interval prior to biting as indicated by a significant elevation compared to baseline Immediately after biting, heart rate dropped precipitously to baseline levels as would be expected according to the tension-reduction hypothesis To rule out simple locomotor explanations of the changes in heart rate, we examined a similar activity (biting the vest) and performed the same heart rate analysis Vest biting was associated with a modest, nonsignificant increase in heart rate with little change in heart rate in the following interval These results suggest that NIB can produce tension reduction which is independent of simple locomotor changes Further research will be aimed at examining stress hormone le vels in monkeys just prior to and after biting as an independent corroboration of the tension reduction hypothesis